What I learned today

[benjiboy]

I learned today how to tell if a polarizing filter that isn't marked on the rim is a linear or a circular one that may be useful to my fellow apugers, because so many A/F cameras these days have semi-silvered mirrors, and need circular ones. If you hold the the filter to your eye with the male thread towards your eye and look into a mirror through it and note the shade of grey, then without changing the filters orientation turn it round 180 degrees so the male thread is on the mirror side of you, if the shade of grey doesn't change it's a linear one.

 

[2F/2F]

Thank you. I don't have any linear ones, but I hope to get one soon for some product shots I will have to do on 4x5. Linear is what the instructor recommended.

I always thought that you could tell the difference because the circular rotates, while the linear slides.

I have a question, though.

What filter factor do you use with a polarizing filter? I ask because as you rotate the front glass while holding one in front of a light meter, the reading changes by almost two stops throughout the revolution.

 

[David A. Goldfarb]

Thank you. I don't have any linear ones, but I hope to get one soon for some product shots I will have to do on 4x5. Linear is what the instructor recommended.

I always thought that you could tell the difference because the circular rotates, while the linear slides.

No, they both rotate. Circular polarizers tend to be more expensive, so that's why you might use a linear polarizer on a camera that doesn't require a circular polarizer. Cameras that use beamsplitting prisms for metering and autofocus cameras generally require circular polarizers.

I have a question, though.

What filter factor do you use with a polarizing filter? I ask because as you rotate the front glass while holding one in front of a light meter, the reading changes by almost two stops throughout the revolution.

Usually 1.5 stops, some two stops. It doesn't matter where the polarizer is set--it's always the same factor.

 

[2F/2F]

Thank you kindly. I love this Website!

 

[johnnywalker]

Usually 1.5 stops, some two stops. It doesn't matter where the polarizer is set--it's always the same factor.

The light built-in light meter in my FM3A changes the shutter speed (when on aperture priority) as you rotate the filter, and the image gets lighter (or darker) in the viewfinder. How can it be the same factor? Or am I misunderstanding what you're saying?

 

[benjiboy]

not that simple

Thank you. I don't have any linear ones, but I hope to get one soon for some product shots I will have to do on 4x5. Linear is what the instructor recommended.

I always thought that you could tell the difference because the circular rotates, while the linear slides.

I have a question, though.

What filter factor do you use with a polarizing filter? I ask because as you rotate the front glass while holding one in front of a light meter, the reading changes by almost two stops throughout the revolution.

The reason I discovered this is that reading the manual for my Canon T90, it says, you must use a circular one with it, because although it isn't A/F it effects the metering, the one I have has had the writing worn off the rim, I now know it's a linear one, and will have to buy the right one.
Filter factors, as David says as a rule of thumb 1.5 stops but you can hold it up to your eye and get the right polarity (in some meters you can programme this correction factor in ), then hold it over the sensor on your light meter before screwing on the lens, I engrave the edges of my filter to try and maintain the setting.
I hope this helps.

 

[MattKing]

The light built-in light meter in my FM3A changes the shutter speed (when on aperture priority) as you rotate the filter, and the image gets lighter (or darker) in the viewfinder. How can it be the same factor? Or am I misunderstanding what you're saying?

It can be hard to see sometimes, but in actuality only parts of the image in the viewfinder darken as you rotate the filter, while others remain unchanged.

The filter factor used for polarizers is that which tends to result in proper exposure for the shadows and the non-specular midtones and highlights. In most cases, those are the parts of the image we want, and which the polarizer has little effect on, whereas the polarizers do remove unwanted glare and reflection.

Glare and reflections tend to distort meter readings, so when we remove them, we tend to improve the accuracy of the reading (unless of course the subject of the photo is the glare or reflection, in which case you probably wouldn't use a polarizer in the first place).

The reason polarizer's require a filter factor is that in doing their work, they also add some neutral density.

Hope this helps.

Matt